Bottom Line:
A number of different sites including the Mn4O5Ca cluster, P680, PheoD1, QA, QB and cytochrome b559 have been hypothesized to produce reactive oxygen species in the photosystem.In this communication using Fourier-transform ion cyclotron resonance mass spectrometry we have identified several residues on the D1 and D2 proteins from spinach which are oxidatively modified and in close proximity to QA (D1 residues (239)F, (241)Q, (242)E and the D2 residues (238)P, (239)T, (242)E and (247)M) and PheoD1 (D1 residues (130)E, (133)L and (135)F).These residues may be associated with reactive oxygen species exit pathways located on the reducing side of the photosystem, and their modification may indicate that both QA and PheoD1 are sources of reactive oxygen species on the reducing side of Photosystem II.

Affiliation: Department of Biological Sciences, Division of Biochemistry and Molecular Biology, Louisiana State University, Baton Rouge, Louisiana, United States of America.

ABSTRACTUnder a variety of stress conditions, Photosystem II produces reactive oxygen species on both the reducing and oxidizing sides of the photosystem. A number of different sites including the Mn4O5Ca cluster, P680, PheoD1, QA, QB and cytochrome b559 have been hypothesized to produce reactive oxygen species in the photosystem. In this communication using Fourier-transform ion cyclotron resonance mass spectrometry we have identified several residues on the D1 and D2 proteins from spinach which are oxidatively modified and in close proximity to QA (D1 residues (239)F, (241)Q, (242)E and the D2 residues (238)P, (239)T, (242)E and (247)M) and PheoD1 (D1 residues (130)E, (133)L and (135)F). These residues may be associated with reactive oxygen species exit pathways located on the reducing side of the photosystem, and their modification may indicate that both QA and PheoD1 are sources of reactive oxygen species on the reducing side of Photosystem II.

pone-0058042-g003: Detail of the Oxidized Residues in the Vicinity of QA.A close-up of the QA – Non-Heme Iron – QB region is shown. The T. vulcanus residues corresponding to the oxidatively modified spinach residues (Table 1) are highlighted and labeled. The D1 protein is shown in pale green and the D2 protein is shown in pale yellow. The oxidatively modified residues of D1 are shown in dark green while those of D2 are shown in orange, with the individual modified residues being labeled. QA is shown in yellow, QB in green and the non-heme iron is shown in bright red.

Mentions:
With respect to the residues in the vicinity of QA in Thermosynechococcus, it should be noted that there is a one amino acid deletion at residue 10 in the D2 sequence with respect to the spinach sequence (Table 1). Consequently, in Thermosynechococcus: QA – 2.9Å – D2:246M – D1:239F – D1:241Q – D1:242E – D2:241E. These residues appear to form a near contiguous chain of oxidized residues leading from QA (Fig. 3). Two additional oxidized residues D2:237P and 238T may also be part of this oxidized residue complex but these are more distantly located. The mass spectra identifying these modified residues are shown in Figs. 1, S1 and S2. All of these residues are at least partially surface-exposed. D2:246M, however, exhibits very limited contact with the bulk solvent. It should be noted that the oxidized D2 residues 237P and 238T are also in relatively close proximity to QB (9 Å and 13 Å, respectively, Fig. 3). While no additional oxidized residues in the immediate vicinity of QB were observed leading to the surface-located residues D2:237P and 238T, it is formally possible that oxidized residues are present in the region near QB, but that they escaped detection in our experiments.

pone-0058042-g003: Detail of the Oxidized Residues in the Vicinity of QA.A close-up of the QA – Non-Heme Iron – QB region is shown. The T. vulcanus residues corresponding to the oxidatively modified spinach residues (Table 1) are highlighted and labeled. The D1 protein is shown in pale green and the D2 protein is shown in pale yellow. The oxidatively modified residues of D1 are shown in dark green while those of D2 are shown in orange, with the individual modified residues being labeled. QA is shown in yellow, QB in green and the non-heme iron is shown in bright red.

Mentions:
With respect to the residues in the vicinity of QA in Thermosynechococcus, it should be noted that there is a one amino acid deletion at residue 10 in the D2 sequence with respect to the spinach sequence (Table 1). Consequently, in Thermosynechococcus: QA – 2.9Å – D2:246M – D1:239F – D1:241Q – D1:242E – D2:241E. These residues appear to form a near contiguous chain of oxidized residues leading from QA (Fig. 3). Two additional oxidized residues D2:237P and 238T may also be part of this oxidized residue complex but these are more distantly located. The mass spectra identifying these modified residues are shown in Figs. 1, S1 and S2. All of these residues are at least partially surface-exposed. D2:246M, however, exhibits very limited contact with the bulk solvent. It should be noted that the oxidized D2 residues 237P and 238T are also in relatively close proximity to QB (9 Å and 13 Å, respectively, Fig. 3). While no additional oxidized residues in the immediate vicinity of QB were observed leading to the surface-located residues D2:237P and 238T, it is formally possible that oxidized residues are present in the region near QB, but that they escaped detection in our experiments.

Bottom Line:
A number of different sites including the Mn4O5Ca cluster, P680, PheoD1, QA, QB and cytochrome b559 have been hypothesized to produce reactive oxygen species in the photosystem.In this communication using Fourier-transform ion cyclotron resonance mass spectrometry we have identified several residues on the D1 and D2 proteins from spinach which are oxidatively modified and in close proximity to QA (D1 residues (239)F, (241)Q, (242)E and the D2 residues (238)P, (239)T, (242)E and (247)M) and PheoD1 (D1 residues (130)E, (133)L and (135)F).These residues may be associated with reactive oxygen species exit pathways located on the reducing side of the photosystem, and their modification may indicate that both QA and PheoD1 are sources of reactive oxygen species on the reducing side of Photosystem II.

Affiliation:
Department of Biological Sciences, Division of Biochemistry and Molecular Biology, Louisiana State University, Baton Rouge, Louisiana, United States of America.

ABSTRACTUnder a variety of stress conditions, Photosystem II produces reactive oxygen species on both the reducing and oxidizing sides of the photosystem. A number of different sites including the Mn4O5Ca cluster, P680, PheoD1, QA, QB and cytochrome b559 have been hypothesized to produce reactive oxygen species in the photosystem. In this communication using Fourier-transform ion cyclotron resonance mass spectrometry we have identified several residues on the D1 and D2 proteins from spinach which are oxidatively modified and in close proximity to QA (D1 residues (239)F, (241)Q, (242)E and the D2 residues (238)P, (239)T, (242)E and (247)M) and PheoD1 (D1 residues (130)E, (133)L and (135)F). These residues may be associated with reactive oxygen species exit pathways located on the reducing side of the photosystem, and their modification may indicate that both QA and PheoD1 are sources of reactive oxygen species on the reducing side of Photosystem II.